current; perhaps all the way to zero or cutoff if the voltage
is high enough. Figure 1 shows a cutaway view of a triode
with an indirectly heated cathode.

Common tube types include tetrodes (two grids) and
pentodes (three grids) which add screen and suppressor
grids to manage secondary effects of the cathode-to-anode
electron flow. Most high power transmitting tubes are
triodes or tetrodes. The audio tube amp’s most common
tube is some variation on the classic 6L6 power-beam
pentode. Figure 2 shows the schematic symbol for a
tetrode.

Tube Action

There are many similarities between the vacuum tube
triode and the solid-state field-effect transistor, or FET.
In particular, the tube acts somewhat like an N-channel
depletion-mode JFET or MOSFET. As grid-to-cathode (
gate-to-source in the FET analog) voltage, VG, increases, so does
cathode-to-anode (source-to-drain) current. There is no
“P-type” vacuum, of course! (Tube voltages such as VG
and VP are assumed with the cathode as the reference.)

Each of the characteristic curves on the graph in Figure
3 shows the behavior of IP at different grid voltages. The
top curve shows that with VG = 0, there is plate current. As
VG becomes more negative, the current curves flatten out
at lower values of IP. This creates a linear region in which
a change in VG (such as from an audio or RF input signal)
creates an equivalent but larger change in IP. Thus, we
have an amplifier. The basic circuit for a triode amplifier is
shown in Figure 4.

The different curves in Figure 3 show that what
matters to controlling the tube’s plate current, IP, is the
voltage between the grid and cathode, VG. A vacuum tube
acts as a voltage-controlled current source, or VCVS. Like
the FET, the relationship between the control voltage and
resulting current is expressed as transconductance — gm = Ip
/ VG — and has the units of siemens (S) which are amps per
volts; more often given as mS, which are mA per volt.

The gain — μ or mu — of the tube depends on the

n FIGURE 3. Characteristic curves for a 6L6 vacuum
tube. Each individual curve shows the plate current
versus plate-to-cathode voltage for one value of the grid-to-cathode voltage. This tube is commonly used in audio
amplifiers.
n FIGURE 4. Simple schematic for a triode amplifier. C1
and C3 are DC blocking capacitors to isolate the AC
input and output from the DC operating voltage on the
grid and plate. R1 and R2 are the grid and cathode bias
resistors, respectively. The bias voltage is developed by
current flowing from the electrode to ground. R3 is the
plate bias resistor, and also sets the amplifier gain. (Graphic
courtesy of the American Radio Relay League.)
n FIGURE 2. The symbol for a tetrode vacuum tube is
very similar to a pictorial of how the tube is constructed.

This symbol shows an indirectly-heated cathode,
electrically isolated from the filament or heater. (Graphic
courtesy of the American Radio Relay League.)